The development of regulatory behaviors necessary for state, motor, and emotion control may have certain underlying physiological processes in the parasympathetic nervous system that play a role in their functioning. One specific psychophysiological correlate of regulatory behavior that has been extensively studied in the existing literature is that of heart rate variability, or vagal tone. Low heart rate variability in children has been associated with maladaptive outcomes (i.e., conduct disorder; Pine et al., 1996) and high heart rate variability has been associated with better social competence (Eisenberg et al., 1995). Thus, it is critical to understand the trajectory of vagal tone from infancy to childhood and how it both influences and is influenced by development. The objectives of the current application are 1) to examine the stability of baseline vagal tone and vagal regulation longitudinally in infants from 3 months to 3 years of age, 2) to examine the concordance between infants' behavioral and vagal responses to challenge tasks, 3) to examine the influence of factors endogenous (i.e., temperament) and exogenous (i.e., maternal sensitivity) to the infant on vagal trajectories over time, 4) to examine factors related to ethnicity and family income as predictors of vagal trajectories and 5) to examine baseline vagal tone and vagal withdrawal trajectories as predictors of socio-emotional functioning assessed by maternal report at 36, 48, and 60 months of age. The Durham Child Health and Development Study has demographic, maternal report, behavioral, and physiological data on 199 infants and their families at seven time points (3, 6, 12, 18, 24, 30, and 36 months). At each visit, infant vagal tone and behavioral response was measured before, during, and immediately following various challenge tasks meant to elicit negative reactivity and regulation from infants. Further, maternal behavior toward her infant during a free play and puzzle task was video recorded for behavioral coding. Our current understanding of vagal tone as a physiological index of regulatory functioning is limited. It is important to move beyond concurrent assessments of vagal response and identify patterns and trajectories that develop over time. The proposed analyses will allow us to examine the development of vagal tone over the first three years as a predictor of typical and atypical socio-emotional outcomes. Relevance to Public Health: The proposed study is designed to provide longitudinal analyses of the development of infant heart rate variability (i.e., vagal tone), and the regulation of vagal tone, across the first three years of life. This physiological system may underlie the processes of emotion regulation and executive functioning and has been associated with both positive and maladaptive behavioral outcomes in children. Understanding the various child and family characteristics that are related to its development will provide new insights into individual differences in such areas as self-regulation of emotion, responses to challenge such as the transition to school, and the acquisition of social competency in peer groups. [unreadable] [unreadable] [unreadable]